Demantoid is the green variety of andradite, which is itself one of the six fundamental sub-species of garnet. This coloration arises from the presence of trace chromium atoms within the mineral’s internal crystal structure, but unwanted additional yellowish or, worse still, brownish components may be observed when iron is also present.
Demantoid’s varietal name was derived from the French word for “diamond”, as both materials display rainbow-like flashes known as dispersion. Unfortunately, a compromise is usually required with regards to the visibility of this phenomenon within a stone and its color-saturation, as dispersion is more clearly seen within the lighter-toned, yellowish-green material typical of recently-discovered Namibian and Madagascan deposits. Conversely, the preferred deep-green hues possessed by the original Russian demantoids tend to somewhat obscure the effect.
However, only Russian material is capable of containing the radiating, fibrous “horsetail” inclusions that can greatly increase a stone’s salability and value.